The present invention relates generally to methodology and apparatus for treatment of sleep apnea and, more particularly, to mono-level, bi-level and variable positive airway pressure apparatus, as well as feedback type versions thereof, including circuitry for enabling a patient to selectively actuate one or more pressure ramp cycles wherein, during each ramp cycle, available airway pressure increases with time from a predetermined minimum pressure value to a prescription pressure, thereby facilitating the patient""s transition from a waking to a sleeping state.
The sleep apnea syndrome afflicts an estimated 1% to 5% of the general population and is due to episodic upper airway obstruction during sleep. Those afflicted with sleep apnea experience sleep fragmentation and intermittent, complete or nearly complete cessation of ventilation during sleep with potentially severe degrees of oxyhemoglobin desaturation. These features may be translated clinically into extreme daytime sleepiness, cardiac arrhythmias, pulmonary-artery hypertension, congestive heart failure and/or cognitive dysfunction. Other sequelae of sleep apnea include right ventricular dysfunction with cor pulmonale, carbon dioxide retention during wakefulness as well as during sleep, and continuous reduced arterial oxygen tension. Hypersomnolent sleep apnea patients may be at risk for excessive mortality from these factors as well as by an elevated risk for accidents while driving and/or operating potentially dangerous equipment.
Although details of the pathogenesis of upper airway obstruction in sleep apnea patients have not been fully defined, it is generally accepted that the mechanism includes either anatomic or functional abnormalities of the upper airway which result in increased air flow resistance. Such abnormalities may include narrowing of the upper airway due to suction forces evolved during inspiration, the effect of gravity pulling the tongue back to appose the pharyngeal wall, and/or insufficient muscle tone in the upper airway dilator muscles. It has also been hypothesized that a mechanism responsible for the known association between obesity and sleep apnea is excessive soft tissue in the anterior and lateral neck which applies sufficient pressure on internal structures to narrow the airway.
The treatment of sleep apnea has included such surgical interventions as uvulopalatopharyngoplasty, gastric surgery for obesity, and maxillo-facial reconstruction. Another mode of surgical intervention used in the treatment of sleep apnea is tracheostomy. These treatments constitute major undertakings with considerable risk of postoperative morbidity if not mortality. Pharmacologic therapy has in general been disappointing, especially in patients with more than mild sleep apnea. In addition, side effects from the pharmacologic agents that have been used are frequent.
Thus, medical practitioners continue to seek non-invasive modes of treatment for sleep apnea with high success rates and high patient compliance including, for example in cases relating to obesity, weight loss through a regimen of exercise and regulated diet.
Recent work in the treatment of sleep apnea has included the use of continuous positive airway pressure (CPAP) to maintain the airway of the patient in a continuously open state during sleep. For example, U.S. Pat. No. 4,655,213 and Australian patent AU-8-83901/82 both disclose sleep apnea treatments based on continuous positive airway pressure applied within the airway of the patient.
Also of interest is U.S. Pat. No. 4,773,411 which discloses a method and apparatus for ventilatory treatment characterized as airway pressure release ventilation and which provides a substantially constant elevated airway pressure with periodic short term reductions of the elevated airway pressure to a pressure magnitude no less than ambient atmospheric pressure.
U.S. Pat. No. 5,199,424 and published PCT Application No. WO 88/10108 describes a CPAP apparatus which includes a feedback system for controlling the output pressure of a variable pressure air source whereby output pressure from the air source is increased in response to detection of sound indicative of snoring. According to additional embodiments of the apparatus disclosed in these references, a pressure ramp cycle (i.e., a gradual increase in output pressure) may occur upon initial activation of the apparatus which gradually increases output pressure from a predetermined minimum to a predetermined maximum or therapeutic pressure specifically selected for the patient.
Publications pertaining to the application of CPAP in treatment of sleep apnea include the following:
1. Lindsay, D A, Issa F G, and Sullivan C. E. xe2x80x9cMechanisms of Sleep Desaturation in Chronic Airflow Limitation Studied with Nasal Continuous Positive Airway Pressure (CPAP),xe2x80x9d Am Rev Respir Dis, 1982; 125: p. 112.
2. Sanders N H, Moore S E, Eveslage J. xe2x80x9cCPAP via nasal mask: A treatment for occlusive sleep apnea, Chest, 1983; 83: pp. 144-145.
3. Sullivan C E, Berthon-Jones M. Issa F G. xe2x80x9cRemission severe obesity-hypoventilation syndrome after short-term treatment during sleep with continuous positive airway pressure, Am Rev Respir Dis, 1983; 128: pp. 177-181.
4. Sullivan C E, Issa F G, Berthon-Jones M., Eveslage J. xe2x80x9cReversal of obstructive sleep apnea by continuous positive airway pressure applied through the nares, Lancet, 1981; 1: pp. 862-865.
5. Sullivan C E, Berthon-Jones M. Issa F G. xe2x80x9cTreatment of obstructive apnea with continuous positive airway pressure applied through the nose. Am Rev Respir Dis, 1982; 125: p. 107. Annual Meeting Abstracts.
6. Rapoport D M, Sorkin B, Garay S M, Goldring R N. xe2x80x9cReversal of the xe2x80x98Pickwickian Syndromexe2x80x99 by long-term use of nocturnal nasal-airway pressure,xe2x80x9d N Engl J. Med, 1982; 307: pp. 931-933.
7. Sanders M H, Holzer B C, Pennock B E. xe2x80x9cThe effect of nasal CPAP on various sleep apnea patterns, Chest, 1983; 84: p. 336. Presented at the Annual Meeting of the American College of Chest Physicians, Chicago Ill., October 1983.
8. Sanders, M H. xe2x80x9cNasal CPAP Effect on Patterns of Sleep Apneaxe2x80x9d, Chest, 1984; 86: 839-844.
Although mono-level positive airway pressure or CPAP has been found to be very effective and well accepted, it suffers from some of the same limitations, although to a lesser degree, as do the surgery options; specifically a significant proportion of sleep apnea patients do not tolerate CPAP well. Thus, development of other viable non-invasive therapies has been a continuing objective in the art.
The present invention contemplates a novel and improved method for treatment of sleep apnea as well as novel methodology and apparatus for carrying out such improved treatment method. The invention contemplates the treatment of sleep apnea through application of pressure at variance with ambient atmospheric pressure within the upper airway of the patient in a manner to promote patency of the airway to thereby relieve upper airway occlusion during sleep.
In a first embodiment of the invention, positive pressure is applied at a substantially constant, patient-specific prescription pressure within the airway of the patient to maintain the requisite patent or xe2x80x9csplintxe2x80x9d force to sustain respiration during sleep periods. This form of treatment is commonly known as mono-level CPAP therapy.
In another embodiment of the invention, pressure is applied alternately at relatively higher and lower prescription pressure levels within the airway of the patient so that the pressure-induced patent force applied to the patients airway is alternately a larger and a smaller magnitude force. The higher and lower magnitude positive prescription pressure levels, which will be hereinafter referred to by the acronyms IPAP (inspiratory positive airway pressure) and EPAP (expiratory positive airway pressure), may be initiated by spontaneous patient respiration, apparatus preprogramming, or both, with the higher magnitude pressure (IPAP) being applied during inspiration and the lower magnitude pressure (EPAP) being applied during expiration. This method of treatment may descriptively be referred to as bi-level therapy. In bi-level therapy, it is EPAP which has the greater impact upon patient comfort. Hence, the treating physician must be cognizant of maintaining EPAP as low as is reasonably possible to maintain sufficient pharyngeal patency during expiration, while optimizing user tolerance and efficiency of the therapy.
This latter embodiment contemplates a novel and improved apparatus which is operable in accordance with a novel and improved method to provide sleep apnea treatment. More specifically, a flow generator and an adjustable pressure controller supply air flow at a predetermined, adjustable pressure to the airway of a patient through a flow transducer. The flow transducer generates an output signal which is then conditioned to provide a signal proportional to the instantaneous flow rate of air to the patient. The instantaneous flow rate signal is fed to a low pass filter which passes only a signal indicative of the average flow rate over time. The average flow rate signal typically would be expected to be a value representing a positive flow as the system is likely to have at least minimal leakage from the patient circuit (e.g., small leaks about the perimeter of the respiration mask worn by the patient). The average flow signal is indicative of such leakage because the summation of all other components of flow over time must be essentially zero since inspiration flow must equal expiration flow volume over time, that is, over a period of time the volume of air breathed in equals the volume of the gases breathed out.
Both the instantaneous flow signal and the average flow rate signal are fed to an inspiration/expiration decision module which is, in its simplest form, a comparator that continually compares the input signals and provides a corresponding drive signal to the pressure controller. In general, when the instantaneous flow exceeds average flow, the patient is inhaling and the drive signal supplied to the pressure controller sets the pressure controller to deliver air, at a preselected elevated pressure, to the airway of the patient. Similarly, when the instantaneous flow rate is less than the average flow rate, the patient is exhaling and the decision circuitry thus provides a drive signal to set the pressure controller to provide a relatively lower magnitude of pressure in the airway of the patient. The patient""s airway thus is maintained open by alternating higher and lower magnitudes of pressure which are applied during spontaneous inhalation and exhalation, respectively.
As has been noted, some sleep apnea patients do not tolerate standard, i.e., mono-level, CPAP therapy. Specifically, approximately 25% of patients cannot tolerate CPAP due to the attendant discomfort. Standard CPAP mandates equal pressures (i.e., a single prescription pressure) during both inhalation and exhalation. The elevated pressure during both phases of breathing may create difficulty in exhaling and the sensation of an inflated chest. However, we have determined that although both inspiratory and expiratory air flow resistances in the airway are elevated during sleep preceding the onset of apnea, the airway flow resistance may be less during expiration than during inspiration. Thus it follows that the bi-level therapy of our invention as characterized above may be sufficient to maintain pharyngeal patency during expiration even though the pressure applied during expiration is not as high as that needed to maintain pharyngeal patency during inspiration. In addition, some patients may have increased upper airway resistance primarily during inspiration with resulting adverse physiologic consequences. Thus, our invention also contemplates applying elevated pressure only during inhalation thus eliminating the need for global (inhalation and exhalation) increases in airway pressure. The relatively lower pressure applied during expiration may in some cases approach or equal ambient pressure. The lower pressure applied in the airway during expiration enhances patient tolerance by alleviating some of the uncomfortable sensations normally associated with CPAP.
Under prior CPAP therapy, pressures as high as 20 cm H2O have been required, and some patients on nasal CPAP thus have been needlessly exposed to unnecessarily high expiratory pressures with the attendant discomfort and elevated mean airway pressure, and theoretic risk of barotrauma. Our invention permits independent application of a higher inspiratory airway pressure in conjunction with a lower expiratory airway pressure in order to provide a therapy which is better tolerated by the 25% of the patient population which does not tolerate CPAP therapy, and which may be safer and more comfortable in the other 75% of the patient population.
As has been noted hereinabove, the switch between higher and lower prescription pressure magnitudes can be controlled by spontaneous patient respiration, apparatus preprogramming, or both. Hence, the manufacturer or the clinician can govern respiration rate and volume or, alternatively, this capability may be independently ascribed to the patient. As has been also noted, the invention contemplates automatic compensation for system leakage whereby nasal mask fit and air flow system integrity are of less consequence than in the prior art. In addition to the benefit of automatic leak compensation, other important benefits of the invention include lower mean airway pressures for the patient and enhanced safety, comfort and tolerance.
In all embodiments, the present invention makes use of xe2x80x9crampxe2x80x9d circuitry operatively connected to pressure control means of the apparatus and selectively activatable by the patient to effect at least one pressure xe2x80x9cramp cyclexe2x80x9d which is described in greater detail below. The maximum duration(s) of the ramp cycle(s), the shape(s) of the ramp curve(s) and the prescription pressure(s) are normally established by a sleep study of the patient and this data can be programmed into the apparatus of the instant invention. It is also desirable that the apparatus be operable either by manual controls located directly on the apparatus or via remote control.
Approximately 25% of all patients who undergo CPAP therapy for sleep apnea experience respiration discomfort and find it difficult to fall asleep because of the therapy. The purpose of a ramp cycle is to alleviate this discomfort. A ramp cycle is an automatic cycle that, once activated, causes the apparatus (mono-level, bi-level or variable) to output a predetermined minimum positive pressure at or above ambient pressure which is gradually increased over a predetermined time period known as xe2x80x9cramp timexe2x80x9d during which the patient begins to fall asleep. Upon expiration of the ramp time the patient typically has fallen asleep and at such time the pressure produced by the apparatus is that of the patient""s therapy prescription pressure(s) whereupon the patient receives normal treatment as he sleeps.
A particular advantage of the present invention is that the unique ramp circuitry enables not only an initial ramp cycle to be achieved for when one first attempts to sleep but such circuitry also permits one or more additional cycles to be selectively activated by the user at instances where the user awakens during an extended rest period, or when the user fails to fall asleep during the first ramp cycle and again requires a ramp cycle to fall back to sleep, or even within (i.e., during) an already ongoing ramp cycle. Typically, during a sleeping period of several hours, the time required to once again fall asleep after briefly being awakened is generally less than the time spent initially falling asleep. To accommodate this phenomenon, the ramp circuitry of the instant invention allows adjustment of the ramp time of any additional ramp cycle to run for a selected fraction of the initial ramp time, which itself may be a pre-programmed, patient-selected a clinician-selected fraction of a prescription time preset by a health care professional in supervision of the patient""s sleep apnea treatment.
The ramp circuitry enables a physician or other health care worker to set the initial ramp time(s) and prescription pressure(s). Additionally, however, the novel ramp circuitry of the present invention permits adjustment of the xe2x80x9cshapexe2x80x9d of the pressure ramp curve, whereby the physician, health care worker or patient can suitably manipulate appropriate controls associated with the ramp circuitry to control the pressure output pattern of the ramp (as represented as a function of pressure versus time) such that it may assume virtually any configuration including, inter alia, linear, stepped, or curvilinear slope, depending upon a patient""s particular needs as dictated by the results of the patient""s sleep study. In the case of bi-level systems according to the instant invention, the ramp circuitry also affords, for example, simultaneous, independent, identical or differential ramping of IPAP and EPAP. Alternatively, the parameters establishable by the ramp circuitry may also be preprogrammed by the manufacturer.
Additionally, sufferers of sleep apnea are sometimes afflicted by other maladies which limit the degree to which they may safely physically exert themselves. An advantage of the present invention is that it enables a limited-mobility user, at his discretion, to operate the apparatus either by manual controls located directly on the apparatus or via remote control. Equally as important, it provides any sleep apnea sufferer using the apparatus with the peace of mind of knowing that the pressure can be reduced at any time via the remote control. Further, the preferred embodiment of the remote control contemplated for use in the present invention is one which the user can operate easily and reliably either in light or darkness to turn the apparatus on and off as well as selectively activate the first or subsequent ramp cycles.
As additional or alternative design features, the apparatus may include an automatic ON/OFF mechanism and/or alternative ramp activation means.
The automatic ON/OFF mechanism desirably comprises a sensor means situated within or proximate to the patient""s breathing circuit. Such breathing circuit will be understood to include, but is not limited to, the gas flow conduit, the gas flow generator means and the respiratory interface, e.g., oral mask, nasal mask, oral/nasal mask, endotracheal tube, nasal cannulae, or other suitable appliance. The sensor means may assume the form of a pressure, flow, thermal, audio, optic, electrical current, voltage, force, displacement or other suitable transducer which detects the presence (and/absence) of the patient. More particularly, according to a first mode of operation, when the respiratory interface is appropriately positioned over the patient""s face, the sensor means will operate so as to detect at least one of the above-mentioned conditions indicative of the patient""s presence and generate a signal that is transmitted to the flow generator to activate the apparatus. In a second mode of operation, the sensor means may be designed solely for apparatus activation purposes. Hence, upon removal of the respiratory interface, the sensor means would fail to detect any conditions indicative of the patient""s presence and, therefore, generate an appropriate signal to deactivate the apparatus. A third mode of operation combines these functions. That is, the sensor means may be operable to detect both the presence and absence of the patient and generate a signal to activate the apparatus upon detection of a condition indicative of the patient""s presence, as well as an apparatus deactivation signal upon failure of detecting such a condition, i.e., the patient""s absence.
The alternative ramp activation means may comprise a sensor means responsive to signals of selected magnitude and/or frequency consciously produced by the patient. In accordance with a presently preferred embodiment, the alternative ramp activations means may comprise a pressure transducer, for instance, a microphone located within or near the patient""s respiratory interface, associated gas flow conduit or gas flow generator and capable of detecting sound of a limited frequency range spanning that associated with human speech. So configured, the transducer would be nonresponsive to common ambient sounds produced by the user (e.g., coughing or sneezing), machinery noise, music or animal sounds. Moreover, by being isolated through its enclosure within the gas flow system, the transducer would detect only the patient""s speech to the exclusion of others in the vicinity or speech emanating from television or radio sources. Upon detection of the patient""s speech such as, for example, when the patient awakens and speaks to start a new ramp cycle, the audio transducer generates and transmits an activation signal to the ramp circuitry to initiate the desired ramp cycle. The sensor may alternatively be operable to begin a ramp cycle in response to detection of a predetermined pattern of inhalations and/or exhalations or other conscious actions by the patient.
As noted hereinabove, the ramp circuitry, remote or automatic ON/OFF mechanisms and/or remote or pressure responsive ramp activation means may be incorporated and utilized with success in mono-level, bi-level and variable positive airway pressure ventilation apparatus or patient-responsive feedback type versions thereof, an example of one version of which will be described in greater detail hereinafter. Other embodiments of the invention employ novel system control circuitry including reset circuitry, safety circuitry, therapy delay circuitry and/or minimum system leakage assurance circuitry which may also be used in conjunction with mono-level, bi-level and variable level ventilation apparatus, but find especially beneficial application with feedback type versions of those ventilation apparatus because of their particular manner of operation.
Briefly, the system reset circuitry permits the patient (if suddenly awakened, for example) to instantaneously reset the system output pressure to a predetermined reduced pressure after which the treatment may then proceed as it did prior to reset. An advantage attendant to both the ramp circuitry and reset circuitry is that they both afford the patient the opportunity to immediately respire against a reduced pressure once awakened, thereby enhancing patient comfort and transition back to a sleeping state. Unlike the ramp circuitry, however, the reset circuitry permits the patient to more rapidly receive the full benefits of the positive airway pressure therapy since therapy resumes instantaneously upon reset. Such modality, as will be appreciated, is more likely to be exercised by a patient who generally experiences little difficulty in falling back to sleep after being awakened during an extended period of sleep;
The safety circuitry of the present invention allows the patient or his overseeing health care professional to establish minimum and maximum system output pressures below and above which the system will not dispense pressurized gas. The minimum pressure will, of course, be at least zero and, preferably, a threshold pressure sufficient to maintain pharyngeal patency during expiration. The maximum pressure, on the other hand, will be some pressure somewhat less than that which would result in over-inflation and perhaps rupture of the patient""s lungs. The safety circuitry functions differently than the prescription pressure controls of the ramp circuitry. That is, instead of establishing lower and upper prescription pressures to be applied during normal usage of the apparatus it sets absolute minimum and maximum fail-safe output pressure limits which will not be exceeded, and therefore potentially cause physical harm to the patient, in the event other system components malfunction.
The therapy delay circuitry proposed herein permits a patient to be diagnosed and treated during a single sleep study. Typically, the practice has been for the patient to undergo two sleep studies at an appropriate observation facility such as a hospital, clinic or laboratory. The first night is spent observing the patient in sleep and recording selected parameters such as oxygen saturation, chest wall abdominal movement, air flow, expired CO2, ECG, EEG, EMG and eye movement. This information can be interpreted to diagnose the nature of the sleeping disorder and confirm the presence or absence of apnea and, where present, the frequency of apneic episodes and extent and duration of associated oxygen desaturation. Apneas can be identified as obstructive, central or mixed.
The second night is spent with the patient undergoing nasal positive airway pressure therapy. When apnea is observed the pressure setting is increased as required to determine the maximum pressure necessary to prevent apnea. As for determining the minimum pressure required to prevent occlusions, for a given patient in a given physical condition there normally will be found different minimum pressures for various stages of sleep. Thus, the appropriate minimum pressure discovered in the laboratory is necessarily the maximum of all minimum pressures recorded for that particular night and.may not necessarily be the ideal minimum pressure for all occasions.
The therapy delay circuitry of the instant invention is operable to suppress the therapy for any desired period of time while associated data storage and retrieval means compile data associated with selected ones of the patient""s physiological parameters to be used for diagnosis of the patient""s particular sleep disorder. Once sufficient data is recorded, system control adjustments based upon the data may be inputted as appropriate and the therapy delay circuitry may be switched from its therapy suppression mode to a therapy application mode, whereby treatment specifically adapted and responsive to the patient""s observed physiological condition may be effectively implemented in a single night in the sleep study facility. With the costs of daily hospital and related medical stays ever on the rise, the advantage of reducing the length of the patient""s sleep study to potentially half of what otherwise would be required is self-evident.
The present invention also provides for minimum system leakage assurance circuitry, the function of which is to assure that the system discharges a minimum leakage flow during therapy, i.e., when the patient is asleep or attempting to fall asleep. The minimum system leakage assurance circuitry desirably comprises an adjustable leakage test pressure control, an adjustable pre-therapy pressure control, an adjustable timer, and a switch. The switch is responsive to the timer and triggers the leakage test pressure control to transmit a signal causing the apparatus to output a preset leakage test pressure. The switch is also operable to trigger the pre-therapy pressure control to transmit a signal causing the apparatus to output a preset pre-therapy pressure. The preset leakage test pressure may be the peak pressure output by the apparatus during the previous night or some other desired pressure, whereas the pre-therapy pressure may be the.minimum pressure output by the apparatus during the previous night or some other pressure.
Using the timer, after the apparatus is, activated the system will output the leakage test pressure to temporarily overpressurize the gas flow circuit for some preselected period of time. This time period will normally be sufficient for the patient to properly position and seal the mask on his face and adjust any gas conduit connections such that system leakage flow is brought to a minimum. After the passage of the prescribed time period, the timer triggers the switch to activate the pre-therapy pressure control, thereby causing the system to output the selected pre-therapy pressure for a specified duration also established by the timer. After expiration of the designated time for pre-therapy pressure application, the timer then urges the switch to assume a neutral position whereby the apparatus outputs its mono-level, bi-level or variable ventilation pressure therapy.
An advantage of such an arrangement is that by temporarily overpressurizing the gas flow circuit prior to treatment, unwanted system leaks can be discovered and sealed before the patient falls asleep. Hence, leakage flow (or average system flow) and its associated pressure can be minimized while the patient sleeps, thereby enhancing patient comfort. In lieu of or addition to the timer, the minimum system leakage assurance circuitry may also include a low leak detector. The low leak detector detects whether the system, under the application the leak test pressure, outputs a leakage flow less than a predetermined minimum. If a sufficiently low leakage flow is detected, the low leak detector automatically overrides the timer and triggers the pre-therapy pressure control to cause the system to output the specified pre-therapy pressure for the preset time and then output the appropriate therapy pressure.
Similarly, in addition to or in lieu of the timer and/or the low leak detector, the minimum system leakage assurance circuitry may include a manual override, e.g., an audio transducer, which responds to patient initiated commands to override the timer in a manner similar to the low leak detector.
Other details, objects and advantages of the present invention will become apparent as the following description of the presently preferred embodiments and presently preferred methods of practicing the invention proceeds.